Planar thin film solar cells (TFSC) dominate the current generation of solar cell technology. The cost and the efficiency of a planar thin film semiconductor are directly related. In other words, the techniques and materials that lead to increased efficiency also lead to increased cost. Many cost reduction techniques used with planar thin film solar cells, such as reducing the thickness of the semiconductor or eliminating materials, tend to lower efficiency, often significantly. Conventional bulk silicon (Si) solar cells have efficiencies in the range of 15% to 25%, while conventional ultrathin solar cells have efficiencies in the range of 8% to 10%. The fundamental reason for the lower efficiency of conventional ultrathin solar cells is the reduction in the amount of light trapped and absorbed resulting from the use of a thinner semiconductor.
In order for the semiconductor of a planar thin film solar cell to trap light, the light must enter the semiconductor at angles large enough to allow the light to be internally reflected. The limit to this trapping ability is often referred to as the 4n2 limit, where n is the refractive index for the semiconductor. Generally, the ability to trap light decreases with the thickness of the semiconductor (i.e., film) in a planar thin film solar cell. In other words, a planar thin film solar cell requires a large film thickness to maximize the amount of light that can be trapped. Conventional planar thin film solar cells commonly have complicated designs, use expensive materials, and require additional processing steps in order to approach the theoretical light trapping limit. The complexity, specialized materials, and additional processing increase the cost of planar thin film solar cells.